Implement application default credentials (#32)

diff --git a/docs/reference/google.auth.rst b/docs/reference/google.auth.rst
index e26862f..6769e1c 100644
--- a/docs/reference/google.auth.rst
+++ b/docs/reference/google.auth.rst
@@ -21,6 +21,7 @@
 
    google.auth.credentials
    google.auth.crypt
+   google.auth.environment_vars
    google.auth.exceptions
    google.auth.jwt